Heat pumps and air conditioners are thermodynamic systems generically referred to as heat engines. These heat engines oppose the normal flow of heat from a warm to a cold body by an input of energy (work). They can provide considerably higher energy efficiency for comfort heating in residential and commercial structures than primary energy heating. However, the second law of thermodynamics provides that no heat engine can be 100% efficient, and this is reflected by losses in the system. These losses, among others, consist of mechanical friction losses, and losses due to the non-ideal nature of the working fluid (refrigerant). These losses can be minimized, and therefore efficiency improved (a measure of efficiency called Coefficient Of Performance (COP) is used in the Heating Ventilating and Air Conditioning (HVAC) industry) by operating the non-ideal refrigerant at optimum conditions.
The presence of contaminants, such as air or moisture, can cause the COP to be severely degraded. Too little or too much refrigerant in the system can also cause the COP to be degraded. A lack of adequate insulation around the vapor line can cause the COP to be degraded, particularly in the heat pump winter heating cycle. Malfunctions in the system, such as a stuck expansion valve or partially blocked capillary expansion tube, can cause the COP to be adversely affected. It is the estimate of an industry expert that 50% of all HVAC systems installed are operating considerably below optimum.
It is the service technician's job to determine what problem may be causing a malfunction or performance degradation. Further, because heat pumps used in forced warm-air heating systems may not provide air which is warm to the human touch, it is frequently the technician's goal to reassure the disgruntled first time heat pump owner that all is well (if it truly is).
However, it is industry experience that large personnel turnover, lack of adequate diagnostic tools and/or difficulty in using available tools, and lack of training in thermodynamic principles many times precludes accurate diagnosis. This results in inefficient operation of HVAC systems in industrial, commercial, and residential applications.
The traditional service tools for the HVAC technician is a set of pressure gages used to measure high and low side system pressure, and temperature gages to measure indoor inlet, indoor outlet, and outdoor temperatures. In addition, a well-equipped technician will have a (sling) hygrometer for measuring relative humidity. Readings obtained from these instruments, coupled with psychrometric tables and manufacturers' performance curves can provide a reasonably accurate system status.
It is rare, however, that all the instruments are available, and that the technician is sufficiently skilled to interpret the readings correctly. Most often, conclusions are drawn from pressure gage readings alone. But pressure gage readings alone can be very misleading. For instance, pressure gage readings can be reasonably normal, accounting for other variables such as humidity, indoor and outdoor temperatures, vapor line insulation, and air flow, and yet the system may be performing well below optimum because of air or moisture contamination (very common) of the refrigerant.